linux-kernel - Re: [PATCH v1.1 1/2] x86/sev: Use per-CPU PSC structure in prep for unaccepted memory support

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Message-ID: <d5902382-70f4-be07-99f6-80d17cdfdcaa@intel.com>
Date:   Wed, 3 Aug 2022 11:17:46 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        "H. Peter Anvin" <hpa@...or.com>,
        Michael Roth <michael.roth@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v1.1 1/2] x86/sev: Use per-CPU PSC structure in prep for
 unaccepted memory support

On 8/3/22 11:11, Tom Lendacky wrote:
> +	/*
> +	 * Use the MSR protocol when either:
> +	 *   - executing in an NMI to avoid any possibility of a deadlock
> +	 *   - per-CPU GHCBs are not yet registered, since __vmgexit_psc()
> +	 *     uses the per-CPU GHCB.
> +	 */
> +	if (in_nmi() || !ghcb_percpu_ready)
> +		return early_set_pages_state(__pa(vaddr), npages, op);
> +
> +	spin_lock_irqsave(&psc_desc_lock, flags);

Would it be simpler to just do a spin_trylock_irqsave()?  You fall back
to early_set_pages_state() whenever you can't acquire the lock.

That avoids even having to know what the situations are where you
_might_ recurse.  If it recurses, the trylock will just naturally fail.
 You simply can't have bugs where the "(in_nmi() || !ghcb_percpu_ready)"
conditional was wrong.